1. Field of the Invention
This invention relates to a method and apparatus for finishing a grooved surface of an article such as a commutator before the grooved surface is cut.
2. Description of the Prior Art
As is well known in the art, it has long been a keen desire to devise a method capable of finishing the grooved surface of an article, e.g., a grooved commutator, a shaft having a keyseat, a bobbin having an axial groove, etc. without formation of objectionable burrs around the grooves when the grooved surface is cut or formed. For purposes of illustration only, the present invention will be described throughout the specification with reference to a commutator.
Commutators are used in various motors such as DC motors having brushes, or other appliances and are composed of a plurality of commutator segments or bars arranged in a drumlike cylinder and supported by a commutator sleeve made of a thermosetting synthetic resin, with air gaps or grooves defined between adjacent ones of the commutator bars and the outer periphery of the commutator sleeve.
In the manufacture of the commutators of the foregoing construction, it is customary practice to form a commutator blank by first filling an internally and circumferentially grooved pipe of an electrically conductive material such as copper with a thermosetting synthetic resin, and then separating the copper pipe into a plurality of circumferentially spaced commutator segments or bars which are electrically separated from one another by undercuts or grooves defined between the adjacent ones of the commutator bars. Then the commutator blank thus produced is finished by cutting or turning the outer peripheral surface thereof so as to provide an improved commutation.
The conventional practice is disadvantageous in that, due to the presence of the grooves between the commutator bars, the turning step is performed interruptedly or non-continuously and hence involves the formation of objectionable burrs projecting from edges of the respective commutator bars. The burrs thus produced are likely to cause an accidental conduction or shortcircuiting between the adjacent commutator bars. To avoid such shortcircuiting, a laborious cleaning of the grooves is necessary after the commutator blank is cut on the outer peripheral surface thereof.
With the foregoing drawbacks in view, somewhat successful methods have been proposed by Japanese Patent Laid-open Publication (JP-A) Nos. 55-58754 and 55-29247. According to the method disclosed in JP-A-55-58754, air gaps or grooves between adjacent commutator bars of a commutator blank are filled with a solvent soluble synthetic resin material which is removed by immersing the commutator blank in a solvent after the commutator blank is cut on its outer peripheral surface. In the method shown in JP-A-55-29247, a commutator blank is composed of a plurality of commutator bars circumferentially spaced by spacer bars made of a solvent soluble synthetic resin material. The spacer bars are also removed by immersing the commutator blank in a solvent after the outer peripheral surface of the commutator is finished.
The proposed methods are still unsatisfactory in that care must be taken both in the disposal of the dissolved synthetic resin and in the handling of the solvent, resulting in a low productivity of the commutator.